Parkinson's disease (PD) is a common age-related neurodegenerative disorder in which multiple aspects of visuospatial cognitive function are impaired. Spatial dysfunction arises from pathological changes in high- order association areas of the brain but also from defective input from lower-level visual processingareas. Deficits in vision are prevalent in PD but virtually nothing is known about the relation between vision, cognition, and daily function in this disorder. Also unknown are the cognitive sequelae of the side of initial motor impairment, despite PD nearly always having unilateral onset of neurodegeneration and unequivocal evidence linking spatial abnormalities to right-hemisphere dysfunction. We propose a multi-level approach to examining visuospatial dysfunction in PD. One major focus is on the visuospatial consequencesof visual deficits, specifically the reduced signal strength that results from contrast sensitivity loss. An innovative aspect of the study is our use of frequency doubling technology and optical coherence tomographyto assess visual (including retinal) function. A second focus is on the side of onset of motor impairment, contralateral o the hemisphere with predominant basal ganglia dysfunction. We plan to conduct control tests of motor- system and prefrontal function to ascertain the extent of independence of the occipital and parietal areas in affecting spatial function in PD. Further, we will conduct an exploration of the relation of visual and visuospatial impairments to daily function as assessed through self-report questionnaires and with an objective open-field measure of gait curvature. Assessmentwill occur at baseline and at one-year follow-up. We propose to examine the ability of 36 patients with left-onset PD (LPD), 36 with right-onset PD (RPD), 36 healthy age-matched adults, and 36 healthy young adults to perform on tests of vision and visuospatial function. Individuals with PD will likely exhibit poorer performance on most tests of vision and visuospatial function than age-matched adults, who in turn will exhibit poorer performance than young adults. We further expect that LPD will show more extensive visuospatial impairments than RPD, and that the pattern of impairments will distinguish the two subgroups. Our tests target visual and visuospatial symptoms reported by the patients themselves, ensuring that the elucidation of mechanisms will have clinical relevance. Demonstration of different profiles of performance in LPD and RPD would bolster arguments of selective visuospatial deficits in PD and suggest differential visual, cognitive, and functional enhancement strategies.